Refinig Process

Refining of Petroleum
Petroleum is a complex mixture of organic liquids called crude oil and natural gas, which occurs naturally in the ground and was formed millions of years ago. Crude oil varies from oilfield to oilfield in colour and composition, from a pale yellow low viscosity liquid to heavy black 'treacle' consistencies.
Crude oil and natural gas are extracted from the ground, on land or under the oceans, by sinking an oil well and are then transported by pipeline and/or ship to refineries where their components are processed into refined products. Crude oil and natural gas are of little use in their raw state; their value lies in what is created from them: fuels, lubricating oils, waxes, asphalt, petrochemicals and pipeline quality natural gas.
An oil refinery is an organised and coordinated arrangement of manufacturing processes designed to produce physical and chemical changes in crude oil to convert it into everyday products like petrol, diesel, lubricating oil, fuel oil and bitumen.
As crude oil comes from the well it contains a mixture of hydrocarbon compounds and relatively small quantities of other materials such as oxygen, nitrogen, sulphur, salt and water. In the refinery, most of these non - hydrocarbon substances are removed and the oil is broken down into its various components, and blended into useful products.
Natural gas from the well, while principally methane, contains quantities of other hydrocarbons - ethane, propane, butane, pentane and also carbon dioxide and water. These components are separated from the methane at a gas fractionation plant.
Petroleum hydrocarbon structures
Petroleum consists of three main hydrocarbon groups:
Paraffins
These consist of straight or branched carbon rings saturated with hydrogen atoms, the simplest of which is methane (CH4) the main ingredient of natural gas. Others in this group include ethane (C2H6), and propane (C3H8).
Hydrocarbons
With very few carbon atoms (C1 to C4) are light in density and are gases under normal atmospheric pressure. Chemically paraffins are very stable compounds.
Naphthenes
Naphthenes consist of carbon rings, sometimes with side chains, saturated with hydrogen atoms. Naphthenes are chemically stable, they occur naturally in crude oil and have properties similar to paraffins.
Aromatics
aromatic hydrocarbons are compounds that contain a ring of six carbon atoms with alternating double and single bonds and six attached hydrogen atoms. This type of structure is known as a benzene ring. They occur naturally in crude oil, and can also be created by the refining process.
The more carbon atoms a hydrocarbon molecule has, the "heavier" it is (the higher is its molecular weight) and the higher is its the boiling point.
Small quantities of a crude oil may be composed of compounds containing oxygen, nitrogen, sulphur and metals. Sulphur content ranges from traces to more than 5 per cent. If a crude oil contains appreciable quantities of sulphur it is called a sour crude; if it contains little or no sulphur it is called a sweet crude.
The refining process
Every refinery begins with the separation of crude oil into different fractions by distillation.
The fractions are further treated to convert them into mixtures of more useful saleable products by various methods such as cracking, reforming, alkylation, polymerisation and isomerisation. These mixtures of new compounds are then separated using methods such as fractionation and solvent extraction. Impurities are removed by various methods, e.g. dehydration, desalting, sulphur removal and hydrotreating.
Refinery processes have developed in response to changing market demands for certain products. With the advent of the internal combustion engine the main task of refineries became the production of petrol. The quantities of petrol available from distillation alone was insufficient to satisfy consumer demand. Refineries began to look for ways to produce more and better quality petrol. Two types of processes have been developed:
breaking down large, heavy hydrocarbon molecules
reshaping or rebuilding hydrocarbon molecules.
Distillation (Fractionation)
Because crude oil is a mixture of hydrocarbons with different boiling temperatures, it can be separated by distillation into groups of hydrocarbons that boil between two specified boiling points. Two types of distillation are performed: atmospheric and vacuum.
Atmospheric distillation takes place in a distilling column at or near atmospheric pressure. The crude oil is heated to 350 - 400oC and the vapour and liquid are piped into the distilling column. The liquid falls to the bottom and the vapour rises, passing through a series of perforated trays (sieve trays). Heavier hydrocarbons condense more quickly and settle on lower trays and lighter hydrocarbons remain as a vapour longer and condense on higher trays.
Liquid fractions are drawn from the trays and removed. In this way the light gases, methane, ethane, propane and butane pass out the top of the column, petrol is formed in the top trays, kerosene and gas oils in the middle, and fuel oils at the bottom. Residue drawn of the bottom may be burned as fuel, processed into lubricating oils, waxes and bitumen or used as feedstock for cracking units.
To recover additional heavy distillates from this residue, it may be piped to a second distillation column where the process is repeated under vacuum, called vacuum distillation. This allows heavy hydrocarbons with boiling points of 450oC and higher to be separated without them partly cracking into unwanted products such as coke and gas.
The heavy distillates recovered by vacuum distillation can be converted into lubricating oils by a variety of processes. The most common of these is called solvent extraction. In one version of this process the heavy distillate is washed with a liquid which does not dissolve in it but which dissolves (and so extracts) the non-lubricating oil components out of it. Another version uses a liquid which does not dissolve in it but which causes the non-lubricating oil components to precipitate (as an extract) from it. Other processes exist which remove impurities by adsorption onto a highly porous solid or which remove any waxes that may be present by causing them to crystallise and precipitate out.
Reforming
Reforming is a process which uses heat, pressure and a catalyst (usually containing platinum) to bring about chemical reactions which upgrade naphthas into high octane petrol and petrochemical feedstock. The naphthas are hydrocarbon mixtures containing many paraffins and naphthenes. In Australia, this naphtha feedstock comes from the crudes oil distillation or catalytic cracking processes, but overseas it also comes from thermal cracking and hydrocracking processes. Reforming converts a portion of these compounds to isoparaffins and aromatics, which are used to blend higher octane petrol.
paraffins are converted to isoparaffins
paraffins are converted to naphthenes
naphthenes are converted to aromatics
e.g.

catalyst
heptane -> toluene + hydrogen
C7H16 -> C7H8 + 4H2
catalyst
cyclohexane -> benzene + hydrogen
C6H12 -> C6H6+ 3H2

Cracking
Cracking processes break down heavier hydrocarbon molecules (high boiling point oils) into lighter products such as petrol and diesel. These processes include catalytic cracking, thermal cracking and hydrocracking.
e.g.
A typical reaction:
catalyst
C16H34 -> C8H18 + C8H16

Catalytic cracking is used to convert heavy hydrocarbon fractions obtained by vacuum distillation into a mixture of more useful products such as petrol and light fuel oil. In this process, the feedstock undergoes a chemical breakdown, under controlled heat (450 - 500oC) and pressure, in the presence of a catalyst - a substance which promotes the reaction without itself being chemically changed. Small pellets of silica - alumina or silica - magnesia have proved to be the most effective catalysts.
The cracking reaction yields petrol, LPG, unsaturated olefin compounds, cracked gas oils, a liquid residue called cycle oil, light gases and a solid coke residue. Cycle oil is recycled to cause further breakdown and the coke, which forms a layer on the catalyst, is removed by burning. The other products are passed through a fractionator to be separated and separately processed.
Fluid catalytic cracking uses a catalyst in the form of a very fine powder which flows like a liquid when agitated by steam, air or vapour. Feedstock entering the process immediately meets a stream of very hot catalyst and vaporises. The resulting vapours keep the catalyst fluidised as it passes into the reactor, where the cracking takes place and where it is fluidised by the hydrocarbon vapour. The catalyst next passes to a steam stripping section where most of the volatile hydrocarbons are removed. It then passes to a regenerator vessel where it is fluidised by a mixture of air and the products of combustion which are produced as the coke on the catalyst is burnt off. The catalyst then flows back to the reactor. The catalyst thus undergoes a continuous circulation between the reactor, stripper and regenerator sections.
The catalyst is usually a mixture of aluminium oxide and silica. Most recently, the introduction of synthetic zeolite catalysts has allowed much shorter reaction times and improved yields and octane numbers of the cracked gasolines.
Thermal cracking uses heat to break down the residue from vacuum distillation. The lighter elements produced from this process can be made into distillate fuels and petrol. Cracked gases are converted to petrol blending components by alkylation or polymerisation. Naphtha is upgraded to high quality petrol by reforming. Gas oil can be used as diesel fuel or can be converted to petrol by hydrocracking. The heavy residue is converted into residual oil or coke which is used in the manufacture of electrodes, graphite and carbides.
This process is the oldest technology and is not used in Australia.
Hydrocracking can increase the yield of petrol components, as well as being used to produce light distillates. It produces no residues, only light oils. Hydrocracking is catalytic cracking in the presence of hydrogen. The extra hydrogen saturates, or hydrogenates, the chemical bonds of the cracked hydrocarbons and creates isomers with the desired characteristics. Hydrocracking is also a treating process, because the hydrogen combines with contaminants such as sulphur and nitrogen, allowing them to be removed.
Gas oil feed is mixed with hydrogen, heated, and sent to a reactor vessel with a fixed bed catalyst, where cracking and hydrogenation take place. Products are sent to a fractionator to be separated. The hydrogen is recycled. Residue from this reaction is mixed again with hydrogen, reheated, and sent to a second reactor for further cracking under higher temperatures and pressures.
In addition to cracked naphtha for making petrol, hydrocracking yields light gases useful for refinery fuel, or alkylation as well as components for high quality fuel oils, lube oils and petrochemical feedstocks.
Following the cracking processes it is necessary to build or rearrange some of the lighter hydrocarbon molecules into high quality petrol or jet fuel blending components or into petrochemicals. The former can be achieved by several chemical process such as alkylation and isomerisation.
Alkylation
Olefins such as propylene and butylene are produced by catalytic and thermal cracking. Alkylation refers to the chemical bonding of these light molecules with isobutane to form larger branched-chain molecules (isoparaffins) that make high octane petrol.
Olefins and isobutane are mixed with an acid catalyst and cooled. They react to form alkylate, plus some normal butane, isobutane and propane. The resulting liquid is neutralised and separated in a series of distillation columns. Isobutane is recycled as feed and butane and propane sold as liquid petroleum gas (LPG).
e.g.
catalyst
isobutane +butylene -> isooctane
C4H10 + C4H8 -> C8H18
Isomerisation
Isomerisation refers to chemical rearrangement of straight-chain hydrocarbons (paraffins), so that they contain branches attached to the main chain (isoparaffins). This is done for two reasons:
they create extra isobutane feed for alkylation
they improve the octane of straight run pentanes and hexanes and hence make them into better petrol blending components.
Isomerisation is achieved by mixing normal butane with a little hydrogen and chloride and allowed to react in the presence of a catalyst to form isobutane, plus a small amount of normal butane and some lighter gases. Products are separated in a fractionator. The lighter gases are used as refinery fuel and the butane recycled as feed.
Pentanes and hexanes are the lighter components of petrol. Isomerisation can be used to improve petrol quality by converting these hydrocarbons to higher octane isomers. The process is the same as for butane isomerisation.
Polymerisation
Under pressure and temperature, over an acidic catalyst, light unsaturated hydrocarbon molecules react and combine with each other to form larger hydrocarbon molecules. Such process can be used to react butenes (olefin molecules with four carbon atoms) with iso-butane (branched paraffin molecules, or isoparaffins, with four carbon atoms) to obtain a high octane olefinic petrol blending component called polymer gasoline.
Hydrotreating and sulphur plants
A number of contaminants are found in crude oil. As the fractions travel through the refinery processing units, these impurities can damage the equipment, the catalysts and the quality of the products. There are also legal limits on the contents of some impurities, like sulphur, in products.
Hydrotreating is one way of removing many of the contaminants from many of the intermediate or final products. In the hydrotreating process, the entering feedstock is mixed with hydrogen and heated to 300 - 380oC. The oil combined with the hydrogen then enters a reactor loaded with a catalyst which promotes several reactions:
hydrogen combines with sulphur to form hydrogen sulphide (H2S)
nitrogen compounds are converted to ammonia
any metals contained in the oil are deposited on the catalyst
some of the olefins, aromatics or naphthenes become saturated with hydrogen to become paraffins and some cracking takes place, causing the creation of some methane, ethane, propane and butanes.
Sulphur recovery plants
The hydrogen sulphide created from hydrotreating is a toxic gas that needs further treatment. The usual process involves two steps:
the removal of the hydrogen sulphide gas from the hydrocarbon stream
the conversion of hydrogen sulphide to elemental sulphur, a non-toxic and useful chemical.
Solvent extraction, using a solution of diethanolamine (DEA) dissolved in water, is applied to separate the hydrogen sulphide gas from the process stream. The hydrocarbon gas stream containing the hydrogen sulphide is bubbled through a solution of diethanolamine solution (DEA) under high pressure, such that the hydrogen sulphide gas dissolves in the DEA. The DEA and hydrogen mixture is the heated at a low pressure and the dissolved hydrogen sulphide is released as a concentrated gas stream which is sent to another plant for conversion into sulphur.
Conversion of the concentrated hydrogen sulphide gas into sulphur occurs in two stages.
Combustion of part of the H2S stream in a furnace, producing sulphur dioxide (SO2) water (H2O) and sulphur (S).
2H2S + 2O2 -> SO2 + S + 2H2O
Reaction of the remainder of the H2S with the combustion products in the presence of a catalyst. The H2S reacts with the SO2 to form sulphur.
2H2S + 2O2 -> 3S + 2H2O
As the reaction products are cooled the sulphur drops out of the reaction vessel in a molten state. Sulphur can be stored and shipped in either a molten or solid state.
Click here to view a flow chart of a refinery .
Refineries and the environment
Air, water and land can all be affected by refinery operations. Refineries are well aware of their responsibility to the community and employ a variety of processes to safeguard the environment.
The processes described below are those used by the Shell refinery at Geelong in Victoria, but all refineries employ similar techniques in managing the environmental aspects of refining.
Air
Preserving air quality around a refinery involves controlling the following emissions:
sulphur oxides
hydrocarbon vapours
smoke
smells
Sulphur enters the refinery in crude oil feed. Gippsland and most other Australian crude oils have a low sulphur content but other crude's may contain up to 5 per cent sulphur. To deal with this refineries incorporate a sulphur recovery unit which operates on the principles described above.
Many of the products used in a refinery produce hydrocarbon vapours. The escape of vapours to atmosphere are prevented by various means. Floating roofs are installed in tanks to prevent evaporation and so that there is no space for vapour to gather in the tanks. Where floating roofs cannot be used, the vapours from the tanks are collected in a vapour recovery system and absorbed back into the product stream. In addition, pumps and valves are routinely checked for vapour emissions and repaired if a leakage is found.
Smoke is formed when the burning mixture contains insufficient oxygen or is not sufficiently mixed. Modern furnace control systems prevent this from happening during normal operation.
Smells are the most difficult emission to control and the easiest to detect. Refinery smells are generally associated with compounds containing sulphur, where even tiny losses are sufficient to cause a noticeable odour.
Water
Aqueous effluent's consist of cooling water, surface water and process water.
The majority of the water discharged from the refinery has been used for cooling the various process streams. The cooling water does not actually come into contact with the process material and so has very little contamination. The cooling water passes through large "interceptors" which separate any oil from minute leaks etc., prior to discharge. The cooling water system at Geelong Refinery is a once-through system with no recirculation.
Rainwater falling on the refinery site must be treated before discharge to ensure no oily material washed off process equipment leaves the refinery. This is done first by passing the water through smaller "plant oil catchers", which each treat rainwater from separate areas on the site, and then all the streams pass to large "interceptors" similar to those used for cooling water. The rainwater from the production areas is further treated in a Dissolved Air Flotation (DAF) unit. This unit cleans the water by using a flocculation agent to collect any remaining particles or oil droplets and floating the resulting flock to the surface with millions of tiny air bubbles. At the surface the flock is skimmed off and the clean water discharged.
Process water has actually come into contact with the process streams and so can contain significant contamination. This water is treated in the "sour water treater" where the contaminants (mostly ammonia and hydrogen sulphide) are removed and then recovered or destroyed in a downstream plant. The process water, when treated in this way, can be reused in parts of the refinery and discharged through the process area rainwater treatment system and the DAF unit.
Any treated process water that is not reused is discharged as Trade Waste to the sewerage system. This trade waste also includes the effluent from the refinery sewage treatment plant and a portion of treated water from the DAF unit.
As most refineries import and export many feed materials and products by ship, the refinery and harbour authorities are prepared for spillage from the ship or pier. In the event of such a spill, equipment is always on standby at the refinery and it is supported by the facilities of the Australian Marine Oil Spill Centre at Geelong, Victoria.
Land
The refinery safeguards the land environment by ensuring the appropriate disposal of all wastes.
Within the refinery, all hydrocarbon wastes are recycled through the refinery slops system. This system consists of a network of collection pipes and a series of dewatering tanks. The recovered hydrocarbon is reprocessed through the distillation units.
Wastes that cannot be reprocessed are either recycled to manufacturers (e.g. some spent catalysts can be reprocessed), disposed of in EPA-approved facilities off-site, or chemically treated on-site to form inert materials which can be disposed to land-fill within the refinery.
Waste movements within the refinery require a "Process liquid, Sludge and Solid waste disposal permit". Wastes that go off-site must have an EPA "Waste Transport Permit".

Composting of bio degradeable waste

Composting is the purposeful biodegradation of organic matter, such as yard and food waste. The decomposition is performed by micro-organisms, mostly bacteria, but also yeasts and fungi.[1] In low temperature phases a number of macro-organisms, such as springtails, ants, nematodes, isopods and earthworms also contribute to the process, as well as soldier fly, fruit flies and fungus gnats. There are a wide range of organisms in the decomposer community.
A biodegradable material is capable of being completely broken down under the action of microorganisms into carbon dioxide, water and biomass. It may take a very long time for some material to biodegrade depending on its environment (e.g. wood in an arid area versus paper in water), but it ultimately breaks down completely. Many contaminating materials not dealt with in common composting are in fact "biodegradeable", and may be dealt with via bioremediation, or other special composting approaches.
Stable bedding after shredding and three weeks of hot aerobic composting.A compostable material biodegrades substantially under specific composting conditions. It is metabolized by the microorganisms, being incorporated into the organisms or converted into humus. The size of the material is a factor in determining compostability, and mechanical particle size reduction can speed the process. Large pieces of hardwood may not be compostable under a specific set of composting conditions, whereas sawdust of the same type of wood may be. Some biodegradeable materials are only compostable under very specific conditions, usually with an industrial process.
Contents [hide]
1 Importance
2 Materials
3 Approaches
3.1 Active (aerobic)
3.2 Passive (anaerobic)
4 In nature
5 Vermicomposting
6 Industrial systems
7 Agriculture
8 See also
9 References
10 External links



[edit] Importance
Composting upcycles organic kitchen and yard waste and manures into an extremely useful humus-like, soil end product, permitting the return of vital organic matter, nutrients, and particularly bacteria, that are vital to plant nutrition to the soil. Managed aerobic composting arranges environmental conditions so they are optimal for the natural processes to take place. There is a popular expression: "compost happens", but it is helpful to engineer the best possible circumstances for large amounts of organic waste to decompose quickly and efficiently, with the greatest conservation of useful nutrients and mass. Uncontrolled composting is when compost "happens", and although that may be functional in some circumstances, as with forest floor detritus, a neglected heap of kitchen and yard wastes will more likely result in "smells happen", or "rodents happen" long before useful compost does.

Long used in subsistence farming and home gardening for creating garden-ready soil, composting is becoming increasingly important and better understood as a tool for reducing municipal solid waste,[4] and reducing the amount of green waste going into landfills. The decomposition of organic material sent to landfills is a principal cause of methane, an important greenhouse gas, making reduction of organic waste being landfilled a key element in the fight against climate change. In suburban and rural areas, much of the organic waste could be removed from the waste stream by promoting home composting, where consumers compost their yard waste and kitchen scraps on their own land, regardless of whether the material is ever actively re-used as "soil".[5] In urban areas with dwellings predominantly lacking individual yard space, there are indoor small scale composting alternatives, such as vermicomposting and bokashi composting.[6]


[edit] Materials

Posters for compost education (European)Composting organisms require four equally important things to work effectively:

Carbon ("C" or carbohydrates), for energy - the microbial oxidation of carbon produces the heat.
High carbon materials tend to be brown and dry.
Nitrogen ("N" or protein), to grow and reproduce more organisms to oxidize the carbon.
High nitrogen materials tend to be green (or colorful, like fruits and vegetables) and wet.[7]
Oxygen, for oxidizing the carbon, the decomposition process.
Water, in the right amounts to maintain activity without causing anaerobic conditions.
Certain ratios of these elements will provide beneficial bacteria with the nutrients to work at a rate that will heat up the pile. In that process much water will be released as vapor ("steam"), and the oxygen will be quickly depleted, explaining the need to actively manage the pile. The hotter the pile gets, the more often added air and water is necessary; the air/water balance is critical to maintaining high temperatures until the materials are broken down. At the same time, too much air or water also slows the process, as does too much C (or too little N).

The most efficient composting occurs with a C:N mix of about 30 to 1. All organics have both carbon and nitrogen, but amounts vary widely, with characteristics noted above (dry/wet, brown/green).[8] Fresh grass clippings have an average ratio of about 15 to 1 and dry autumn leaves about 50 to 1 depending on species. Mixing equal parts by volume approximates the ideal C:N range. Few individual situations will provide the ideal mix of materials at any point in time - in this respect, home composting is like horseshoes, perfect is great, but close still works. Observation of amounts, and consideration of different materials[9] as a pile is built over time can quickly achieve a workable technique for the individual situation.


Home compost barrel in the Escuela Barreales, Chile.Ingredients that are primarily carbon include:

Dry, straw-type material, such as cereal straws and corn stalks
Dry leaves (best shredded, as with a rotary mower, to prevent matting)
Wood, as coarse or fine (may compact) sawdust, or ground wood waste
Paper, corrugated cardboard, box board, and office paper or newsprint (soy-based inks)
Ingredients with relatively high nitrogen content include:

Green plant material, like crop residues, hay (especially alfalfa), grass clippings and weeds.
Manure from poultry, and herbivorous animals such as horses, cows and llamas.
Kitchen waste - fruit and vegetable cooked waste and trimmings, juicing-pulp residue, tea and coffee grounds.
For "back yard" composting, mixing the materials as they are added increases the rate of decomposition, as does reduced particle size (ie, chopped, shredded), or materials can be added in alternating layers, about 15 centimeters (6 in) thick. Keeping a carbon "cache" handy to the pile for covering and mixing with fresh wet additions (lawn clippings, kitchen scraps) is simplest. Special additions or activators are not necessary, although some sprinklings of good garden loam as a first pile is built will aid more rapid working by inoculating it with beneficial soil bacteria, and some of the material from the first finished batch can be used in the subsequent mixes. Agricultural lime is not necessary - the bacteria prefer a slightly acidic pH, and their processing invariably results in a near-neutral product.[10] Seaweed meal, rock dust or rock flour, and other trace element amendments are best added to the finished compost, or directly to the garden.


[edit] Approaches

[edit] Active (aerobic)

An active compost heap, steaming on a cool morning. The heap is kept warm by the exothermic action of the bacteria as they decompose the organic matter.Hot thermophilic composting is essential with some materials, such as meat and other animal products, dairy products, eggs, grease, cooking oil, manure of non-herbivores, and residuals from the treatment of wastewater, in order to kill pathogens; but these materials are not generally recommended in home composting because of the likelihood of creating odors and attracting rodents. Human waste can be composted by industrial methods as well as composting toilets. When high temperatures are reached, the resulting compost can be safely used for agricultural or horticultural purposes, providing local health regulations are met. Humanure fertilizer (as opposed to night soil) is used throughout the developing world and is becoming more accepted as a garden amendment in the developed world.

Hot, aerobic composting is conducted at close to the ideal conditions noted above, allowing thermophilic bacteria to thrive. These aerobic bacteria break down material faster, producing less odor, fewer pathogens, and less greenhouse gas than cool, uncontrolled, or accidental anaerobic methods. Commercial scale composting operations actively control the composting conditions (C:N ration, moisture level and air), usually in a closed environment (in-vessel composting, tunnel composting or aerated static pile composting), where air is fan forced through the mass, and moisture added with sprayers, or conserved via the enclosure, with computer monitored probes detecting conditions.

In Thailand an aerated static pile system is in use by farmer groups at over 400 sites. The process needs only 30 days to finish without manual turning, with 10 metric tons of compost produced per month. A 38 centimetres (15 in) squirrel-cage blower with 2.2 kilowatts (3.0 hp) motor is used to force air through 10 covered static piles of compost twice a day. The raw materials consist of agricultural wastes and animal manure in the ratio of 3:1 by volume.[11]

High temperatures destroy insects, larvae, and weed seeds, but no compost will be totally sterilized by high temperatures alone. In a hot compost where the temperature exceeds 55 °C (131 °F) for several days, the ability of most organisms to survive is compromised, and there are temperature standards[12] set by various regulating authorities for commercial products[13] Nevertheless, many organisms in nature can survive extreme temperatures, including extremophiles such as Thermus thermophilus which play an important role in thermogenic composting,[14] as well as pathogens such as Clostridium. The necessary second stage of hot composting is maturation, a period allowing the dissipation of any phyto-toxins[15] remaining from the process or contaminating ingredients (eg: chemical residues), and achieving a state of nutrient stability (low C:N ratio) that will not have an impact on Nitrogen availability in the receiving soil.[16]


An alternative to manual turning, the "Wing Ding" compost aeratorFor backyard composters, carbon and nitrogen ratios in various ingredients and the calculations required to get the ideal mixture can be intimidating, so rules of thumb exist for approximating it by ingredient types and condition, as mentioned in Materials above. If the pile is built in a short period, and has a good mix of materials (C:N) and a coarse structure, with about 50% moisture ("like a squeezed out sponge"), the temperature should rise within days to as high as 60 °C (140 °F). When the temperature begins to fall, more air is needed, usually added by turning the pile or using an agitating tool, and moisture may be needed at the same time. Turning or other aeration is usually needed about every 6–10 days to maintain the highest heat levels until the material is fairly uniformly broken down to unrecognizability, and temperatures no longer rebound. A pile that has been maintained at peak temperatures may be ready for maturing in as little as 30 days. Another 30–60 days maturing should suffice to allow passing the "germination test".[17]

To achieve thermophilic decomposition, a compost bin is best about 1 cubic metre (1.3 cu yd), or 1 metre (3 ft) wide, 1 metre (3 ft) tall, and as long as desired for windrow composting. This provides enough insulating mass to build up heat but also allows oxygen infiltration. The center of the pile heats up the most, so regular turning/mixing is needed for insuring all material spends some time in the hottest area. When turning the pile results in no further temperature rise, the active aerobic phase is complete, and the mass may be turned out to a maturing pile. When the matured material has a dark brown crumbly appearance and the smell of rich damp earth, it is ready to use.


Turning a home compost binThe natural sequence of the decomposition community involved will be:

0-15 °C (32-59 °F) - psychrophiles predominate, beginning the heating process as they multiply
15-40 °C (60-104 °F) - mesophiles take over, psychrophiles die off or are relegated to the borders
40-70 °C (105-160 °F) - thermophiles work at their peak, including consuming many other bacteria
At the lower temperatures and around the borders, there will also be various fungal activity, as well as larger organisms, getting their share - a very dry, cooler pile may be attractive to ants, and gastropods may visit very wet piles. As the temperature returns to ambient at the end of the process, the sequence reverses, including new organisms that prefer the more degraded materials. Added heat and pile insulation may be useful in the coldest weather, but is not ordinarily necessary, and is not desirable if it interferes with aeration or natural convective evaporation. Keeping the top dry and burying fresh additions in the center of a pile will be effective during winter conditions until heating resumes in spring.


[edit] Passive (anaerobic)
Cool or ambient temperature composting, when the level of physical intervention is minimal, usually results in temperatures never reaching above 30°C (86°F). It is slower but effective, and is the more common type of composting in domestic gardening. Such composting systems may be in open or closed containers of wood or plastic, or in open exposed piles. Kitchen scraps are put in the garden compost bin and left untended. This scrap bin can have a very high water content which reduces aeration, and may become odorous. To improve drainage and airflow, and reduce odor, carbon-rich materials, or 'browns', such as wood chips, shredded bark, leaves, or twigs may be added to mix and cover each wet addition, or holes made occasionally in the pile. The amount of attention may vary from none through occasional to "regular".


[edit] In nature
An unusual form of composting in nature is seen in the case of the mound-builders (megapodes) of the Australasian region. These Megapodes are fowl-sized birds famous for building nests in the form of compost heaps containing organic litter, in which they incubate their eggs. The male birds work assiduously to maintain the correct incubation temperatures, by adding and removing litter from the compost pile.

Forest floor detritus is the natural fertilizing material in temperate forests. Soil dwelling organisms slowly decompose the continuously deposited litter from below, returning the nutrients to the soil for bacterial conversion to forms useful to the native vegation. This is a natural form of mulching or sheet composting. While some may be tempted to gather this rich, partially decomposed litter for use in a home garden, it is best left to the use of the forest.


[edit] Vermicomposting
Worm composting or vermicomposting is a method using Red Wiggler worms in a container to process kitchen waste. Moistened high-carbon bedding such as shredded paper is used as a base to which the food waste is added, and the worms and micro-organisms convert the materials to rich compost called worm castings, a nutrient and microbially rich material. Worm composting can be done indoors, allowing year-round composting, and providing apartment dwellers with a means of recycling food waste.


[edit] Industrial systems
Industrial composting systems are increasingly being installed as a waste management alternative to landfills, along with other advanced waste processing systems. Mechanical sorting of mixed waste streams combined with anaerobic digestion or in-vessel composting, is called mechanical biological treatment, increasingly used in developed countries due to regulations controlling the amount of organic matter allowed in landfills. Treating biodegradable waste before it enters a landfill reduces global warming from fugitive methane; untreated waste breaks down anaerobically in a landfill, producing landfill gas that contains methane, a potent greenhouse gas.

Large-scale composting systems are used by many urban centers around the world. Co-composting is a technique which combines solid waste with de-watered biosolids, although difficulties controlling inert and plastic contamination from Municipal solid waste makes this approach less attractive. The world's largest MSW co-composter is the Edmonton Composting Facility in Edmonton, Alberta, Canada, which turns 220,000 tonnes of residential solid waste and 22,500 dry tonnes of biosolids per year into 80,000 tonnes of compost. The facility is 38,690 meters2 (416,500 ft2), equivalent to 4½ Canadian football fields, and the operating structure is the largest stainless steel building in North America, the size of 14 NHL rinks.[18]


[edit] Agriculture
Compost is an important source of nutrients commonly used in modern agriculture. Through steaming compost can be sanitized and prepared for further use.